Proper bone healing and subsequent favorable bone remodeling are dependent on maintaining stability between bone fragments, and on maintaining physiologic strain levels. Successful bone graft procedures commonly require an osteoconductive matrix providing a scaffold for bone in-growth, osteoinductive factors providing chemical agents that induce bone regeneration and repair, osteogenic cells providing the basic building blocks for bone regeneration by their ability to differentiate into osteoblasts and osteoclasts, and a substantially stable implant site. Current bone graft materials include autografts, allografts, and a variety of artificial or synthetic bone substitute materials.
For structural bone repair materials to be conveniently used, they must be capable of being formed into complex shapes that are designed to fit the contours of the repair site. Accurately contoured grafts enhance the integration of the natural bone and provide better load carrying capability. Intimate, load-carrying contact often is required between the natural bone and the bone substitute material to promote bone remodeling and regeneration leading to incorporation of the graft by host bone. A general overview of orthopedic implantable materials is given in Damien, Christopher J., and Parsons, Russell J., “Bone Graft and Bone Graft Substitutes: A Review of Current Technology and Applications”, Journal of Applied Bio Materials, Volume 2, pp. 187-208 (1991).
Bone substitute materials have found particular use in the repair of lower back disc deterioration, and a method and a device for such repair is disclosed in Kuslich, U.S. Pat. Nos. 5,549,679 and 5,571,189, respectively. These patents describe a surgical operation in which a bore is drilled laterally into a deteriorated disc body, the bore being enlarged into the bony vertebral bodies above and below the disc to form an enlarged, desirably rounded cavity. The physician then inserts a flexible fabric bag into the cavity and fills the bag with a particulate bone substitute material. The preferred fill material is identified as finely chopped cortical or cancellous bone chips for fusion, hydroxyapatite or similar biocompatible materials, or connective tissue when a fibrous union is desired. Once the bag is packed full, its mouth is closed off, and surgical access to the site is repaired in the usual fashion.
In connection with the procedure described in the above patents, experiments have been performed to replace the cortical or cancellous bone chips with other particulate materials, including particularly ceramic beads. The beads may be formed of zirconia, alumina, hydroxyapatite, or other ceramic material, and may be generally cubic in shape with the sharp edges of the cubes rounded off. As ceramic beads of this type are packed tightly within a fabric bag, the beads may grate against each other, generating fine particulates, as they seek relatively stable positions with respect to each other. Moreover, the beads themselves may actually break when subjected to packing forces. Even when packed tightly, the beads still may move slightly with respect to one another in response to shifting loads until bone in-growth stabilizes their positions. It is desirable to inhibit such movement also, inasmuch as such movement may create local configurations of high stress, leading to bead fracture.
It would be desirable to provide ceramic beads that are resistant to relative movement and fracture when packed together, as, for example, in a fabric bag according to the teachings of the above patents, and which moreover may include osteoconductive and osteoinductive materials such as bone morphogenic protein to foster bone ingrowth.